In many contexts, especially in flue gas cleaning, electrostatic precipitators (ESP) are highly suitable dust collectors. Their design is robust and they are very reliable. Moreover, they are most efficient. Degrees of separation above 99.9% are not unusual. Since, when compared with fabric filters, their operating costs are low and the risk of damage and stoppage owing to functional disorders is considerably smaller, they are a natural choice in many cases. In an electrostatic precipitator, the polluted gas is conducted between electrodes connected to a high-voltage rectifier. Usually, this is a high-voltage transformer with thyristor control on the primary side and a rectifier bridge on the secondary side.
This arrangement is connected to the ordinary AC mains and thus is supplied at a frequency which is 50 or 60 Hz.
The power control is effected by varying the firing angles of the thyristors. The smaller the firing angle, i.e. the longer conducting period, the more current supplied to the precipitator and the higher the voltage between the electrodes of the precipitator.
Modern power supplies for these ESP are so-called series loaded resonant converters (SLR) which allow to have high-power (typically in the range of 10-200 kW) and high-voltage (50-150 kV DC) while at the same time keeping switching losses at a minimum. The focus of the R&D is higher output power.
The used topology is a series loaded resonant converter, SLR, e.g. as given in FIG. 1. The three phase mains with the three phases 1-3 which can be individually switched by switches 4, is rectified by a six-pulse rectifier 6 e.g. comprising diodes 5. This rectifier may however also be an actively switched rectifier. The rectified voltage is smoothed by a DC-link capacitor 13 in the DC-link 6. The DC link voltage is fed to a transistor bridge 8 (H-bridge), comprising four transistors 14, 14′, 15, 15′. The output of the bridge 8 (high frequency AC voltage) is connected, via a resonant tank 9, to the primary of a transformer 10. The resonant tank 9 comprises an inductor 16 and a capacitor 17 in series and together with the primary winding 18 these elements are basically defining the resonance frequency of the resonant tank, which correspondingly can only reasonably be operated around this resonance frequency. The transformer 10, consisting of the primary winding 18 and the secondary winding 19, adapts the input voltage (mains) to the load 12 (ESP, 50-150 kV). The secondary alternating voltage of the transformer 10 is rectified by a high voltage rectifier 11 and fed to the load 12. The output voltage is normally negative.
The power flow in such a topology can be controlled either by varying the frequency of the bridge 8 or by varying the duty ratio of the voltage source (bridge output). Switching frequencies in the vicinity of the resonance of the tank are within normal operation.
Resonant power converters contain resonant L-C networks whose voltage and current waveforms vary sinusoidally during one or more subintervals of each switching period.
These sinusoidal variations are large in magnitude, and the small ripple approximation does not apply. The chief advantage of resonant converters is the reduced switching loss (zero-current switching, zero-voltage switching). Turn-on or turn-off transitions of semiconductor devices can occur at zero crossings of tank voltage or current waveforms, thereby reducing or eliminating some of the switching loss mechanisms. Hence resonant converters can operate at higher switching frequencies than comparable pulse width modulation converters. Zero-voltage switching also reduces converter-generated electromagnetic impulses, and zero-current switching can be used to commutate silicon controlled rectifiers. In specialized applications, resonant networks may be unavoidable, so in high voltage converters there is a significant transformer leakage and inductance and winding capacitance leads to resonant network.
There are however also disadvantages to series or parallel resonant tanks. For example the performance can be optimized at one operating point, but not with a wide range of input voltage and load power variations. Further significant currents may circulate through the tank elements, even when the load is disconnected, leading to poor efficiency at light load. Quasi-sinusoidal waveforms exhibit higher peak values than equivalent rectangular waveforms. These considerations lead to increased conduction losses, which can offset the reduction in switching loss. Resonant converters are usually controlled by variation of switching frequency. In some schemes, the range of switching frequencies can be very large
In order to increase the power handling capability and to establish a scaleable design, modularizing is used. The fundamental issue when modularizing is to control the load sharing i.e., to secure that different modules take equal or well defined shares of the load.